Some forms of containers are subject to unwanted pressure increases. Batteries contain elements that may generate increased pressures under certain conditions. The conditions under which the pressure increase may arise can occur at any time during the life span of the article, or even after its usefulness is over. In some extreme conditions the pressure increase can even lead to an explosive release of pressure. Such an event is to be avoided.
Therefore, it is desireable to provide a venting means to relieve the unwanted pressure increase. The current methods used by most manufacturers involve the use of "vents" that open when the pressure inside the article reaches a pre-determined level. Most of the vents are essentially parts of the container and have a lower stress level than surrounding parts.
The areas of lower, or reduced stress level are created in a variety of ways. In Shillady, et al, U.S. Pat. No. 3,074,602, the area of reduced stress is accomplished by a place or spot having a reduced cross section on a point on the can that would deflect upon pressure build up. The deflection would cause the spot, with the reduced cross section, to rupture, creating a vent for the pressure contents. The problem with this pressure relief device is that the container must be at or near the explosive point before the device triggered. Often, by that point the vent by itself is not enough to prevent an explosion. This problem could not be solved by reducing the cross section even further, because this may result in premature venting.
More recently, the concept of a "reduced cross section" area being used as a vent has been further refined. In Zundel, U.S. Pat. No. 3,831,822, the pressure relief area is "defined by a pair of spaced weakened areas that are separated from each other by unweakened areas". Here the unweakened areas tend to give when explosive conditions arise. This creates tensile stress across the weakened areas, causing them to rupture, creating a vent for the increased pressure. The increased pressure could cause the unweakened areas to rupture very soon after the weakened areas, causing a force that may cause the article to become a projectile.